i915: initialize CADL in opregion

i915: initialize CADL in opregion

This is rather a hack to fix brightness hotkeys on a Clevo laptop. CADL is not
used anywhere in the driver code at the moment, but it could be used in BIOS as
is the case with the Clevo laptop.

The Clevo B7130 requires the CADL field to contain at least the ID of
the LCD device. If this field is empty, the ACPI methods that are called
on pressing brightness / display switching hotkeys will not trigger a
notification. As a result, it appears as no hotkey has been pressed.

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=45452
Tested-by: Peter Wu <lekensteyn@gmail.com>
Signed-off-by: Peter Wu <lekensteyn@gmail.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>